1. Field of the Invention
The present invention pertains to a method for printing and decorating packaging materials, such as paper, paper board, metals, and various flexible and rigid polymer films by electron beam exposure of plural layers of curable inks, which do not substantially change viscosity during the printing process. The inks are essentially free of volatile components such as organic solvents, inorganic solvents, water and low molecular weight fractions before, during and after exposure to electron beam irradiation.
2. Description of the Related Art
Multicolor printing and decorating processes used in manufacturing packaging materials typically require the sequential printing of single-color ink layers. When high-quality image reproduction is desired, it is important to avoid the mixing of a previously applied ink layer with a subsequently applied ink layer. Such layer mixing typically leads to visual defects of printed image. The art of printing has addressed this problem in a number of different ways. The usual way to prevent undesirable color mixing is to dry or cure each applied ink layer prior to the application of a next ink layer. This method, while effective, suffers from a major disadvantage in that it requires a complete drying after the application of each consecutive ink layer. Drying takes time and energy to accomplish. As a result, productivity is reduced and production costs increase. An alternative to drying can be found in flexographic printing with ultra-violet (UV) curable inks. UV inks are free from violate diluents and each ink layer is cured individually prior to applying consecutive ink layers. During this process, each ink layer undergoes a significant viscosity change from a relatively low 500-2000 cps to that of an indefinitely high (solid polymer). This process allows for a stable, high quality printing but requires the introduction low molecular weight acrylated monomers and low molecular weight volatile photoinitiators. Such technology is not well suited for the low odor and migration requirements of food and/or pharmaceutical packaging. U.S. Pat. No. 6,772,683 introduces a process of wet trapping of sequential layers where energy curable flexographic inks contain 5% to 50% of volatile diluents such as water. Loss of water through evaporation leads to the development of viscosity and tack gradients between earlier applied and following ink layers. Even a small loss of water, e.g. a 5% loss of water can cause a 10-fold increase in viscosity during the printing process. A major disadvantage of this process is that the continuous loss of water during the printing process also leads to rheological instability of the printing ink during the entire printing cycle that can be as long as 6-8 hours or longer. These rheological changes (higher viscosity, loss of flow) create problems such as ink starvation, leakage and misting. Delivering this ink to the printing surface requires a very sophisticated and expensive apparatus with multiple pumps and viscosity controls.
In lithographic offset printing, inks are initially formulated free from volatile diluents, but due to mixing with fountain solutions, are required to maintain a hydrophilicity of the non-image area on the lithographic printing plate. As a result, lithographic inks always contain between 5% and 15% of water (fountain solution) that uncontrollably changes ink viscosity depending on the state of the emulsion formed between the ink and the fountain solution. These viscosity variations make lithographic offset printing of multicolor images quite challenging and difficult to control. There is clearly a need for a simple printing and decorating method which would allow applying sequential ink layers that would not substantially change viscosity during the multicolor printing process and not require complicated and costly individual curing or drying units. Such inks should meet all major requirements of the packaging applications, including low odor and migration necessary for food pharmaceutical or other items which are sensitive to migratable residuals from its packaging.
There is a great need for new methods of printing and decorating packaging materials that have a very high visual image quality, yet meeting high fitness for use standards in respect to product resistance and compliance with the U.S. Food and Drug Administration (FDA), and other regulatory agencies worldwide whose requirements have been established to protect foods from alteration. The art of packaging printing and decoration is dominated by liquid ink processes that are based on drying or curing individual ink layers through evaporation of water or volatile organic compounds. These processes consume high amount of energy and often negatively affect the environment due to emission of solvent of greenhouse gases in atmosphere. Many attempts to introduce energy curable solutions, such as Ultraviolet (UV) and electron beam (EB) flexography or UV and EB litho-offset printing processes help to reduce emissions, but either complicate printing processes or generate unacceptable for food and pharmaceutical packaging low molecular weight compounds such as photoinitiators and their by-products that can alter food or other contents through migration. It is possible, as this invention suggests, to print and decorate packaging materials with EB curable printing inks that essentially do not change in viscosity during the entire printing process, making the process very robust and easy to control, and essentially free from any low-molecular weight compounds that can alter odor, off-taste food or migrate from the cured film. Therefore, according to this invention, a multi-color printing process can be accomplished by the application of sequential layers of electron beam curable inks that have viscosity of about 5000 cps or higher, that do not substantially change during the printing process. These inks are essentially free from volatile low molecular weight organic compounds before, during and after exposure to electron beam irradiation. After curing, these inks essentially do not contain migratable species at a level that exceeds 50 parts per billion (ppb) as required by the U.S. Food and Drug Administration (FDA) for food packaging materials. Electron beam curable inks of the invention are a mixture of a colorant and an ethylenically unsaturated component. The process of invention can be used for printing and decoration of various packaging materials.